This application is generally in the area of shape memory polymers, and more particularly to shape memory polymers having enhanced performance characteristics and more than one shape in memory.
Shape memory is the ability of a material to remember its original shape, either after mechanical deformation (FIG. 1), which is a one-way effect, or by cooling and heating (FIG. 2), which is a two-way effect. This phenomenon is based on a structural phase transformation.
The first materials known to have these properties were shape memory metal alloys (SMAs), including TiNi (Nitinol), CuZnAl, and FeNiAl alloys. The structure phase transformation of these materials is known as a martensitic transformation. These materials have been proposed for various uses, including vascular stents, medical guidewires, orthodontic wires, vibration dampers, pipe couplings, electrical connectors, thermostats, actuators, eyeglass frames, and brassiere underwires. These materials have not yet been widely used, in part because they are relatively expensive.
Scientists are actively developing shape memory polymers (SMPs) to replace or augment the use of SMAs, in part because the polymers are light, high in shape recovery ability, easy to manipulate, and economical as compared with SMAs. In the literature, SMPs are generally characterized as phase segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition temperature of the hard segment.
When the SMP is heated above the melting point or glass transition temperature of the hard segment, the material can be shaped. This (original) shape can be memorized by cooling the SMP below the melting point or glass transition temperature of the hard segment. When the shaped SMP is cooled below the melting point or glass transition temperature of the soft segment while the shape is deformed, that (temporary) shape is fixed. The original shape is recovered by heating the material above the melting point or glass transition temperature of the soft segment but below the melting point or glass transition temperature of the hard segment. In another method for setting a temporary shape, the material is deformed at a temperature lower than the melting point or glass transition temperature of the soft segment, resulting in stress and strain being absorbed by the soft segment. When the material is heated above the melting point or glass transition temperature of the soft segment, but below the melting point (or glass transition temperature) of the hard segment, the stresses and strains are relieved and the material returns to its original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabilities of a material are the shape recovery of the original shape and the shape fixity of the temporary shape.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress, particularly at the melting point or glass transition temperature of the soft segment. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above the melting point or glass transition temperature of the soft segment. Also, the hardness of the material changes dramatically when the soft segment is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature of the soft segment, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles, and can be heated above the melting point of the hard segment and reshaped and cooled to fix a new original shape.
The shape memory effect exists for polymers (e.g. heat-shrinkable films). However, it is not a specific bulk property, but results from the polymer""s structure and morphology. The effect is persistent in many polymers, which might differ significantly in their chemical composition. However only a few shape memory polymer systems have been described in the literature (Kim, et al., xe2x80x9cPolyurethanes having shape memory effect,xe2x80x9d Polymer 37(26):5781-93 (1996); Li et al., xe2x80x9cCrystallinity and morphology of segmented polyurethanes with different soft-segment length,xe2x80x9d J. Applied Polymer 62:631-38 (1996); Takahashi et al., xe2x80x9cStructure and properties of shape-memory polyurethane block copolymers,xe2x80x9d J. Applied Polymer Science 60:1061-69 (1996); Tobushi H., et al., xe2x80x9cThermomechanical properties of shape memory polymers of polyurethane series and their applications,xe2x80x9d J. Physique IV (Colloque C1) 6:377-84 (1996)).
Examples of polymers used to prepare hard and soft segments of SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; U.S. Pat. No. 5,665,822 to Bitler et al.; and Gorden, xe2x80x9cApplications of Shape Memory Polyurethanes,xe2x80x9d Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, SMST International Committee, pp. 115-19 (1994). The SMPs that have been developed thus far appear to be limited to being able to hold only one temporary shape in memory. It would be advantageous to provide SMPs that are able to form objects which are able to hold more than one shape in memory.
It is therefore an object of the present invention to provide SMPs that are able to form objects which are able to hold more than one shape in memory.
It is another object of the present invention to provide SMPs with physical and chemical properties and chemical structures which are different than those in conventional SMPs.
It is still another object of the present invention to provide SMPs with shapes in memory that are elicited by a stimulus other than temperature.
Shape memory polymer compositions, articles of manufacture thereof, and methods of preparation and use thereof are described. In a preferred embodiment, the shape memory polymer composition can hold more than one shape in memory. For example, the composition can include a hard segment and at least two soft segments. The Ttrans of the hard segment is at least 10xc2x0 C., and preferably 20xc2x0 C., higher than the Ttrans of one of the soft segments, and the Ttrans of each subsequent soft segment is at least 10xc2x0 C., and preferably 20xc2x0 C., lower than the Ttrans of the preceding soft segment. A multiblock copolymer with a hard segment with a relatively high Ttrans and a soft segment with a relatively low Ttrans can be mixed or blended with a second multiblock copolymer with a hard segment with a relatively low Ttrans and the same soft segment as that in the first multiblock copolymer. Since the soft segments in both multiblock copolymers are identical, the polymers are miscible in each other when the soft segments are melted. The resulting blend has three transition temperatures: one for the first hard segment, one for the second hard segment, and one for the soft segment. Accordingly, these materials are able to memorize two different shapes.
Any polymers that are crystalline or amorphous and that have a Ttrans within the range defined herein can be used to form the hard and soft segments. The melting point or glass transition temperature (hereinafter, the Ttrans) of the hard segment is at least 10xc2x0 C., and preferably 20xc2x0 C., higher than the Ttrans of the soft segment. The Ttrans of the hard segment is preferably between xe2x88x9230 and 270xc2x0 C., and more preferably between 30 and 150xc2x0 C. The ratio by weight of the hard segment:soft segments is between about 5:95 and 95:5, preferably between 20:80 and 80:20.
In some embodiments, the shape memory polymers contain at least one physical crosslink (physical interaction of the hard segment) or contain covalent crosslinks instead of a hard segment. The shape memory polymers also can be interpenetrating networks or semi-interpenetrating networks. In addition to changes in state from a solid to liquid state (melting point or glass transition temperature), hard and soft segments may undergo solid to solid state transitions, and can undergo ionic interactions involving polyelectrolyte segments or supramolecular effects based on highly organized hydrogen bonds.
Articles of manufacture can be prepared from the shape memory polymer compositions, for example, by injection molding, blowing, extrusion, and laser ablation. To prepare an object having a shape in memory, the object can be formed at a temperature above the Ttrans of the hard segment, and cooled to a temperature below the Ttrans of the soft segment. If the object subsequently is formed into a second shape, the object can be returned to its original shape by heating the object above the Ttrans of the soft segment and below the Ttrans of the hard segment.
Articles of manufacture with two or more shapes in memory can be prepared by forming a polymer composition with a hard segment, a first soft segment, and a second soft segment, where the first soft segment has a Ttrans at least 10xc2x0 C. below that of the hard segment and at least 10xc2x0 C. above that of the second soft segment. After the composition is shaped at a temperature above the Ttrans of the hard segment, it can be cooled to a temperature below that of the Ttrans of the first soft segment and above that of the second soft segment and formed into a second shape. The composition can be formed into a third shape after it has been cooled below the Ttrans of the second soft segment. The composition can be heated above the Ttrans of the second soft segment to return the composition to the second shape. The composition can be heated above the Ttrans of the first soft segment to return the composition to the first shape. The composition can also be heated above the Ttrans of the hard segment, at which point the composition loses the memory of the first and second shapes and can be reshaped using the method described above.
Thermoset polymers can be prepared by pre-shaping macromonomers, for example, by extrusion, and fixing the original shape at a temperature above the Ttrans of the thermoset polymer, for example, by photocuring reactive groups on the macromonomer. The original shape, however, can only be programmed one time.
In a preferred embodiment, the shape change occurs in response to a change in temperature. In another embodiment, however, the composition can change its shape in response to application of light, changes in ionic concentration and/or pH, electric field, magnetic field or ultrasound. For example, a SMP can include at least one hard segment and at least one soft segment, wherein at least two of the segments, preferably two soft segments, are linked to each other via a functional group that is cleavable under application of light, electric field, magnetic field or ultrasound. The temporary shape is fixed by crosslinking the linear polymers. By cleaving those links the original shape can be recovered. The stimuli for crosslinking and cleaving these bonds can be the same or different.